1. Field of the Disclosure
The present invention is related to power converters. More specifically, examples of the present invention are related to power converters that operate from an ac input voltage.
2. Background
Controllers for off-line power converters often must measure the ac input voltage to perform functions such as under-voltage detection, over-voltage detection, and fast reset from a momentary loss of input voltage. Since the ac voltage varies periodically between a peak positive value and a peak negative value at the frequency of the power line, the ac line voltage is typically represented numerically as a quantity that is proportional to the peak voltage. For example, the common ac power line voltage referred to as 120 volts is a root mean square (rms) value of a sine wave that is obtained from the magnitude of the peak voltage (169.7 volts) divided by the square root of two. An ac voltage of 120 volts rms is equivalent to a dc voltage of 120 volts when both are applied to the same resistive load such as an incandescent lamp. Controllers for power converters that are required to operate in particular ways within specified limits of an ac input voltage generally must determine the magnitude of the peak of the ac input voltage.
Circuits that measure an input voltage typically do so by using a potential divider across the input voltage that provides a known fraction of the input voltage that is low enough for the circuit to handle. In order to reduce power consumption, the components of the divider are selected to take no more current than necessary from the input. To reduce power consumption further and to reduce the number of components, a current that represents the input voltage can be used instead of a potential divider. However, the current needs to be large enough to guarantee a reliable measurement in the presence of noise. The power taken from the source of input voltage is proportional to the product of the voltage and the current. Since the peak value of the ac input may be hundreds of volts, even the smallest current acceptable for reliable measurement can still result in a significant loss of power, especially when the power converter has a light load or no load. Power converters need a controller that can sense the ac power line reliably with low power consumption.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.